Multiple-stage operational amplifiers (opamps) typically include a cascade of one or more gain stages and an output driver stage for driving an output load. The output stage is, for example, a Class AB amplifier that provides high low-frequency gain. To achieve an overall high open loop gain (e.g. greater than 150 dB), a multiple-stage opamp normally requires three or more gain stages.
The design of multiple-stage opamps with three or more gain stages presents significant design challenges. For example, to achieve unconditional stability, a relatively complex nested Miller frequency compensation scheme must often be used. In the nested Miller frequency compensation scheme, as each new gain stage is added to the system, an additional nested Miller capacitor is added between the opamp output and the inputs to the amplifier of the previous stage in the cascade to produce pole-splitting. For example, in a three-stage amplifier, a first feedback capacitor is provided between the opamp output and the input to the last stage and a second feedback capacitor is provided between the opamp output and the input to the second stage in the cascade. In addition, increasing the complexity of the circuitry, nested Miller compensation also disadvantageously reduces the overall opamp bandwidth and increases the load on the opamp output thereby imposing increased power requirements on the last stage.
In order to design and fabricate less complicated, smaller, and less expensive opamps, an alternative technique to the nested Miller compensated scheme is required. This technique should provide high opamp gain while maintaining stability, and should be suitable for low power opamp applications.